\begin{table}%t1
        \caption{Observations of PSF calibration fields. 
        The Strehl ratio is that measured on-axis in the short exposure
        frames taken before and after the deep science exposures.}
\label{calibtab}
%\centering
\par
        \begin{tabular}{l c c c c}
        \hline 
        \hline 
        Field & Date & Filter &  Exp. Time & Strehl \\
         &  &        &  (min) & (\%) \smallskip \\
        \hline 
        NGC 6809 & 15.12.02 & $K_{\rm s}$ & 60 & 28 \\
        NGC 6752 & 06.09.03 & $K_{\rm s}$ & 12 & 27 \\
        \hline
        \end{tabular}
\end{table}